Science Behind Radiofrequency
In 2002, the FDA approved the first monopolar RF device for facial wrinkle reduction (ThermaCool; Thermage, Inc., Hayward, Calif.). Since 2002, more sophisticated RF devices have been developed to deliver RF energy in different manners (ie, bipolar, multipolar, and fractional) with more safety features. Unlike lasers, RF does not target specific chromophores by selective photothermolysis. Instead, RF generates heat as a result of different tissue resistance or impedance to the electromagnetic current. This means that heat is produced when the tissues' inherent resistance converts the electrical current to thermal energy as dictated by the following formula (Ohm's law): Energy (J) = Current2 × Resistance × Time. For example, adipose tissue has a high tissue impedance and will generate more heat than muscle which has lower impedance for a given amount of time. In fact, when RF energy is directed to subdermal adipose tissue, it has been shown to generate temperatures 7-fold higher than those generated by the dermis, leading to fat necrosis with epidermal preservation.
When RF energy is applied to the underlying skin and soft tissue, it generates contraction by 2 mechanisms: (1) immediate cleavage of hydrogen bonds in the collagen triple helix causing shortening and thickening of the collagen fibrils and (2) initiation of a wound healing cascade to trigger neoangiogenesis, neocollagenesis, and elastin reorganization over the following 3–4 months. This dual mechanism was shown 15 years ago when the ThermaCool TC RF device pilot study treated bovine tendon and human abdominal skin. Electron microscopy evaluation of RF-heated collagen further demonstrates breakage of intramolecular bonds in the collagen fibrils, leading to increased diameter and shortening. Also messenger ribonucleic acid studies show upregulation of collagen gene expression after treatment with RF to the skin.
Clinical studies and animal studies demonstrate that subdermal temperatures from 65°C to 68°C and skin surface temperatures ranging from 38°C to 42°C are required to obtain optimal contraction. Further, it is postulated that heated fibroblasts may be stimulated to produce collagen. Importantly, if temperatures exceed a critical heat threshold, there is the potential for collagen ablation and full-thickness injury.[17,19] There is no single shrinkage temperature of collagen contraction. Rather, the delivery of RF energy is a function of time and temperature to allow for maximal epidermal protection while optimally heating the dermal collagen. For example, studies suggest that for millisecond exposures, the shrinkage temperature is above 85°C, whereas for exposures of several seconds, the shrinkage temperature is in the range of 60°C–65°C (2–15). For every 5°C decrease in temperature, a 10× increase in time is required to achieve a comparable collagen contraction. RF volumetric dermal heating is favorable because it avoids targeting specific dermal targets and instead leverages various tissue impedances to generate desired heat and contraction. This nonspecificity means that RF is safe to use in all Fitzpatrick skin types.
Plast Reconstr Surg Glob Open. 2020;8(8):e2861 © 2020 Lippincott Williams & Wilkins